1. Field of the Invention
This invention relates to an image heating device applied to a copier, a printer or the like, and more particularly, to a device for heating a film utilizing electromagnetic induction.
2. Description of the Related Art
Japanese Utiliy Model Laid-Open Application (Kokai) No. 51-109737 (1976) discloses an induction-heating fixing device for heating a fixing roller utilizing Joule heat by inducing a current therein by a magnetic flux. Since the fixing roller can be directly heated by utilizing the generation of an induced current, a fixing processing having higher efficiency than a heating roller using a tungsten halogen lamp is achieved.
In the electromagnetic-induction heating device disclosed in Japanese Utility Model Laid-Open Application (Kokai) No. 51-109737 (1976), since the energy of an AC magnetic flux generated by an exciting coil is used for raising the temperature of the entire fixing roller, radiation loss is large and the ratio of the fixing energy to the input energy is low, thereby causing an inferior efficiency.
In order to overcome the above-described problems in such an electromagnetic-induction heating device, a method for obtaining high-density thermal energy for fixing by disposing an induction heating unit in the vicinity of a fixing nip portion and using a low-heat-capacity resistor (an electromagnetic-induction heating member made of a magnetic conductive material) having the shape of a cylindrical film, such as a nickel electrodeposited film, as a heater.
In this fixing device, however, the following new problems also arise. That is, when using the above-described cylindrical film as the heater, a supporting member for supporting the cylindrical film from the inside is required in order to provide a sufficient strength to resist a pressing force necessary for fixing. Since the inner surface of the cylindrical film rubs the supporting member, the driving torque increases, and wear or degradation tends to occur.
If the metallic layer made of nickel or the like is present on the inner surface of the cylindrical film, the metallic layer rubs the supporting member, thereby facilitating the occurrence of wear.
In order to overcome such problems, a resin layer may be provided on the inner surface of the cylindrical film for the purpose of increasing wear resistance. In general, a releasing layer made of a fluororesin or the like is formed on the surface of the cylindrical film in order to prevent offset at fixing. Hence, uniform cylindrical resin layers must be provided in a state of facing both surfaces of the resistor layer (metallic layer) serving as the heater. However, the provision of such cylindrical films causes an increase in the production cost and in the heat capacity, and therefore in the rise time. In addition, since the resin layer on the inner surface moves together with the resistor layer, the rate of contribution of thermal energy stored in the resin layer to the heating of an image is reduced, thereby degrading thermal efficiency.